Most case based reasoning (CBR) systems that have been developed in the last couple of decades follow the transformational analogy approach. There are only a few engineering applications of derivational analogy in the published literature. This paper describes a new CBR system called KnowPrice that uses derivational analogy. The software was developed in collaboration with a Swiss construction management company. KnowPrice estimates probability distributions of project costs using cases containing details of past projects. Details related to representation, retrieval and reuse of cases are described. KnowPrice is being used to estimate project costs using real data and is an example of successful partnership between researchers and industry. © Civil-Comp Press, 2005.